V8 3.19 API changes

@laverdet
Hi Marcel, 

I had to make a few tweaks to the code to get it to compile with V8 3.19. I had to do this to bench against generators. I was testing with Andy Wingo's node fork: https://github.com/andywingo/node/tree/v8-3.19).

I have conditioned the code with a NEWAPI flag. I wanted to set this flag from the v8.h definitions but I could not find any version number in this header. Maybe the flag could be passed by the build but I did not want to fool around with it.

Also, I could get the code to compile and run ok but there are still warnings about `New` being deprecated.
 
BTW, fibers did pretty well in the bench: https://gist.github.com/bjouhier/5554200